Such a type of window covering is also known as a blind. The window covering material is formed here by pleated or cellular curtains. Such a window covering is applied for instance in roof lights or in the sloping roof panels of conservatories. Although such window covering are usually installed obliquely, they are also used horizontally or vertically. A characteristic of blinds is that they have at least three profiles. A first profile and a second profile, or the upper and lower profile in an application which is other than horizontal, are mounted fixedly on or in the window frame, while a third profile, which is movable, is placed therebetween. The material of the curtain lies between the upper profile and the movable profile and/or between the lower profile and the movable profile, depending on the configuration. It is also possible to provide covering material between the upper profile and the movable profile as well as between the lower profile and the movable profile. One or more support cords or tensioning cables can be provided to support the window covering material and optionally the movable profile. Some window covering material will sag without support cords. The window covering is most commonly rectangular, although other forms (triangular or trapezoidal) of window coverings are also known.
An operating unit moves the movable profile reciprocally by means of pull cords engaging on the movable profile.
Many existing operating units operate on the basis of friction wherein a continuous cord is wound several times round a roll and brought to tension. Friction between the roll and the cord provides transfer of movement. The more friction present during reciprocating movement of the movable profile, the more drive energy is lost. This phenomena makes battery operated units less suitable in battery-powered drive systems. Operating units are further known wherein use in made of one or more wind-up shafts. Although these encounter less friction and are therefore more suitable for the application of battery-powered systems, these known operating units do however have such large dimensions that they are not suitable, or at least less so, for relatively narrow and long windows. In addition, the existing operating units quite often meet with problems during winding up and/or subsequent unwinding of said pull cords.
It is therefore an object of the present invention to provide an operating unit for window coverings of the above stated type, wherein at least one of the above stated drawbacks or other problems associated with the prior art are obviated.